The invention relates to a process for preparing salts of dialkylphosphinic acids and to the use of the dialkylphosphinate salts prepared by this process.
Aluminum salts of organic phosphorus-containing acids are known as flame retardants. They can be prepared by various processes.
EP-A-0 299 922 describes a process for preparing aluminum salts of phosphoric and phosphonic esters by reacting aluminum with phosphoric acid, phosphonic acid or an ester thereof.
In the process described in EP-A-0 245 207, aluminum compounds are reacted with alkylphosphonic diesters to give the corresponding aluminum salt.
According to EP-A-0 327 496, the reaction of aluminum hydroxide with alkylphosphonic diesters in the absence of water at approximately 180xc2x0 C. likewise leads to aluminum salts of phosphnic half-esters.
EP-A-0 699 708 describes flame-retardant polyester molding compounds, the polyesters being given a flame-retardant finish by adding calcium salts or aluminum salts of phosphinic or diphosphinic acids. The above-mentioned salts are obtained by reacting the corresponding dialkylphosphinic acids with calcium hydroxide or aluminum hydroxide.
DE 24 47 727 describes low-flammability polyamide molding compounds which comprise a salt of a phosphinic acid or of a diphosphinic acid.
However, the abovementioned processes have the disadvantage that the suitable organic phosphorus compounds must first be prepared in a laborious manner. This applies, in particular, to the dialkylphosphinic acids, whose aluminum salts give the best results in the application as flame retardants, and for which, likewise, some synthetic pathways are described.
Thus DE 21 00 779 A1 describes a process for preparing alkyl dialkylphosphinates by addition of olefins having from 2 to 22 carbon atoms to alkylphosphonous esters.
In this case also, there has been the lack to date of an economic synthesis method which leads to homogeneous products in a high yield.
The object therefore underlying the invention is to provide a process for preparing salts of dialkylphosphinic acids in which, in a particularly simple and economical manner, not only the dialkylphosphinic acids and/or their alkali metal salts, but also the desired end products, that is to say dialkylphosphinic salts of certain metals, may be prepared.
This object is achieved by a process of the type described at the outset, which comprises
a) reacting alkylphosphonous and/or hypophosphorous acid and/or alkali metal salts thereof with olefins in the presence of a free-radical initiator to give dialkylphosphinic acids and/or alkali metal salts thereof and
b) reacting the dialkylphosphinic acids and/or alkali metal salts thereof obtained according to a) with metal compounds of Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, Sr, Mn, Li, Na and/or K to give the metal dialkylphosphinate salts.
Preferably, as free-radical initiator, use is made of azo compounds.
Preferably, the azo compounds are cationic and/or non-cationic azo compounds.
Preferably, as cationic azo compounds, use is made of 2,2xe2x80x2-azobis(2-amidinopropane)dihydrochloride or 2,2xe2x80x2-azobis(N,Nxe2x80x2-dimethyleneisobutyramidine)dihydrochloride.
Preferably, as non-cationic azo compounds, use is made of azobis(isobutyronitrile), 4,4xe2x80x2-azobis(4-cyanopentanoic acid) or 2,2xe2x80x2-azobis(2-methylbutyronitrile).
Preferably, as free-radical initiator, use is made of inorganic peroxide free-radical initiators and/or organic peroxide free-radical initiators.
Preferably, as inorganic peroxide free-radical initiator, use is made of hydrogen peroxide, ammonium peroxodisulfate and/or potassium peroxodisulfate.
Preferably, as organic peroxide free-radical initiators, use is made of dibenzoyl peroxide, di-tert-butyl peroxide and/or peracetic acid.
A wide selection of suitable free-radical initiators is found, for example, in Houben-Weyl, Supplementary Volume 20, in the chapter xe2x80x9cPolymerisation durch radikalische Initiierungxe2x80x9d [Polymerization by Free-Radical Initiation] on pages 15-74.
Preferably, the metal compounds are metal oxides, metal hydroxides, metal hydroxide oxides, metal sulfates, metal acetates, metal nitrates, metal chlorides and/or metal alkoxides.
Particularly preferably, the metal compounds are aluminum hydroxide or aluminum sulfates.
Preferably,
a) alkali metal salts of hypophosphorous acid are reacted with olefins in the presence of a cationic free-radical initiator to give the alkali metal dialkylphosphinates and
b) the alkali metal dialkylphosphinates obtained according to a) are reacted with aluminum compounds to give the aluminum dialkylphosphinate salts.
Instead of the alkali metal salts of hypophosphorous acid, an aqueous solution of the free acid can likewise be used without problems.
Preferably, the product mixture obtained according to step a) is reacted with the metal compounds without further purification.
In a further embodiment of the process, the product mixture obtained according to step a) is worked up and thereafter only the dialkylphosphinic acids and/or alkali metal salts thereof obtained according to step a) are reacted with the metal compounds.
Preferably, as olefins, use is made of unbranched or branched xcex1-olefins.
Preferably, as olefins, use is made of those having an internal double bond, cyclic or open-chain dienes and/or polyenes having from 2 to 20 carbon atoms.
Preferably, as olefins, use is made of ethylene, n-propylene, isopropylene, n-butene, isobutene, n-pentene, isopentene, n-hexene, isohexene, n-octene, isooctene, 1-decene, 1,5-cyclooctadiene, 1,3-cyclopentadiene, dicyclopentadiene and/or 2,4,4-trimethylpentene isomer mixture.
Preferably, the olefins bear a functional group.
Suitable olefins are compounds of the formula 
where R1-R4 can be identical or different and are hydrogen, an alkyl group having from 1 to 18 carbon atoms, phenyl, benzyl or alkyl-substituted aromatics.
Suitable olefins are likewise cycloolefins of the formula 
in particular cyclopentene, cyclohexene, cyclooctene and cyclodecene.
Use can also be made of open-chain dienes of the formula 
where R5-R10 are identical or different and are hydrogen or a C1-C6 alkyl group and R11 is (CH2)n where n=0 to 6. Preference is given in this case to butadiene, isoprene and 1,5-hexadiene.
Preferred cyclodienes are 1,3-cyclopentadiene, dicyclopentadiene and 1,5-cyclooctadiene, and also norbornadiene.
Preferably, the alkylphosphonous acid and/or alkali metal salts thereof are methylphosphonous acid or methylphosphonous acid and/or alkali metal salts thereof.
Preferably, the reaction in step a) is carried out at a temperature of from 40 to 130xc2x0 C.
Particularly preferably, the reaction in step a) is carried out at a temperature of from 70 to 110xc2x0 C.
Preferably, the reaction in step b) is carried out at a temperature of from 20 to 150xc2x0 C.
Particularly preferably, the reaction in step b) is carried out at a temperature of from 80 to 120xc2x0 C.
Preferably, the reactions in step a) and in step b) are carried out in an acetic acid medium.
In step b), preference is also given to reaction in aqueous medium.
In this case, the reaction in step b) is carried out after adjusting to a pH range for the salt precipitation which is optimum for the respective system of dialkylphosphinic acid/metal compound.
The present invention also relates in particular to a process in which sodium hypophosphite is reacted with ethylene in the presence of a cationic or non-cationic free-radical initiator or in the presence of a peroxide free-radical initiator to give the sodium salt of diethylphosphinic acid as main product.
This product is then reacted according to the invention with aluminum hydroxide or an aluminum sulfate to give the aluminum salt of diethylphosphinic acid.
The invention also relates to the use of the metal, dialkylphosphinate salts prepared by the process according to the invention for preparing flame retardants.
In particular, the invention relates to the use of the metal dialkylphosphinate salts prepared according to the invention for preparing flame retardants for thermoplastic polymers such as poly(ethylene terephthalate), poly(butylene terephthalate), polystyrene or polyamide and for thermosetting plastics.
Finally, the invention also relates to the use of metal dialkylphosphinate salts prepared by the process according to the invention as additives in polymeric molding compounds.